Problem: Multiply the following complex numbers, marked as blue dots on the graph: $[2(\cos(\pi) + i \sin(\pi))] \cdot [2(\cos(\frac{5}{4}\pi) + i \sin(\frac{5}{4}\pi))]$ (Your current answer will be plotted in orange.)
Answer: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $2(\cos(\pi) + i \sin(\pi))$ ) has angle $\pi$ and radius $2$ The second number ( $2(\cos(\frac{5}{4}\pi) + i \sin(\frac{5}{4}\pi))$ ) has angle $\frac{5}{4}\pi$ and radius $2$ The radius of the result will be $2 \cdot 2$ , which is $4$ The sum of the angles is $\pi + \frac{5}{4}\pi = \frac{9}{4}\pi$ The angle $\frac{9}{4}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{9}{4}\pi - 2 \pi = \frac{1}{4}\pi$ The radius of the result is $4$ and the angle of the result is $\frac{1}{4}\pi$.